Endothelin (ET) is a vasoconstriction peptide hormone, important to the homeostasis and regulation of the biological functions of the cardiovascular system. ET is found not only in the endothelium but also in many other tissues and cell types (Barton et al., 2008, Can. J. Physiol. Pharmacol. 86:485-498). ET is a 2,400 Da peptide of 21 amino acids, having 2 disulfide bonds at its N-terminus, linking the 1st and 15th cysteine residues and the 3rd and 11th cysteine residues, respectively. Its C-terminus contains hydrophobic amino acid residues. Its N-terminal structure is important for binding to its receptor, while its C-terminal structure is important as to where on the receptor to bind. ET has three isoforms: ET-1, ET-2 and ET-3. They differ by a few amino acid residues. ET-1 plays a major role in the regulation of the biological functions of the cardiovascular system. Upon stimulation, endothelial cells synthesize and release ET-1. ET-1 is mainly regulated at the transcription level.
Endothelin receptors (ETR) has two isoforms: ETAR and ETBR, which belong to the G protein coupled receptor (GPCR) family. Upon stimulation, ETAR activates membrane Na+/Ca2+ exchanger (NCX) and Na+/H+ exchanger (NHE) to increase cellular Ca2+ concentrations and to sensitize muscle fibers to Ca2+, resulting in the constriction of vascular smooth muscle and cardiac muscle (Neylon, 1999, Clin. Exp. Pharmacol. Physiol. 26:149-153). Unlike ETAR, ETBR mainly relaxes the vascular smooth muscle cells and cardiac muscle cells (Nelson et al., 2003, Nat. Rev. Cancer 3:110-113).
Pulmonary artery hypertension (PAH) is due to the vasoconstriction of the lung or lung related vasculature, resulting in lung artery insufficiency and a compensatory increase in the blood pressure of the heart. On the microscopic scale, there appear to be changes in the small pulmonary arteries, including intimal fibrosis, medial hypertrophy, and plexiform lesions, causing in situ thrombosis of elastic and small pulmonary arteries, and resulting in increased blood circulation resistance in the whole lung vasculature (Simonneau et al., 2004, J. Am. Coll. Cardiol. 43:5S-12S; Barst et al., 2004, J. Am. Coll. Cardiol. 43:40S-47S). It has been shown that an ETAR antagonist can effectively block the vasoconstrictive signaling of ET-1, ameliorate PAH symptoms, and improve exercise capability and hemodynamics in PAH patients (Serasli et al., 2010, Recent Pat. Cardiovasc. Drug Discov. 5:184-95).
The present invention provides antibodies specifically binding to ETAR, which can inhibit ET-1 signaling. The antibodies can be used as a monotherapy or combination therapy to treat PAH. The antibodies can also be used in diagnostic applications.